Definitely do NOT pour in the old brake fluid!
Your best course of action is to siphon suck out as much fluid as you can from the reservoir before doing your pads. Replace pads one at a time, do not remove both pads from the same caliper and then push the piston in. The pressure will push the opposing caliper too far out and require caliper disassembly. Just one pad at a time.
Once the new pads are in place and everything buttoned up, suck out the remaining fluid from the reservoir and fill with fresh fluid, then flush the brakes at all four wheels and the clutch if it is a manual transmission car. The entire brake hydraulic system should be flushed annually. If you do this, the hydraulics will most likely never need any attention.
BTW, if the brakes have not been flushed in awhile, run about a quart of fluid through them. Then if you flush them annually after that, you can probably flush adequately with something less than a quart.